Pull the respective pulley... pop off the plastic retaining ring... examine the bearings... bad? Replace with new bearings... press in some grease (by hand) reinstall the plastic retaining ring... reinstall the pulley.
Bearings good, upon inspection? Remove the bearings... spray clean the bearings and the pulley... reinstall the bearings... regrease... reinstall the ring... close it up and reinstall the pulley.
Go driving.
